The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used for the storage and retrieval of digital data.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as magnetic field sensors. Digital data memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital symbols as alternative states of magnetization in magnetic materials provided in each memory storage cell, the result being memories which use less electrical power and do not lose information upon removals of such electrical power.
Such memory cells, and magnetic field sensors also, can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant xe2x80x9cflux closure,xe2x80x9d i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This result is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin. Similar xe2x80x9csandwichxe2x80x9d structures are also used in magnetic sensors.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended xe2x80x9csandwichxe2x80x9d structures, and adding possibly alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a xe2x80x9cgiant magnetoresistive effectxe2x80x9d being present in some circumstances. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude or more greater than that due to the well known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response, varying the difference occurring between the direction of the magnetization vector in a ferromagnetic thin-film and the direction of sensing currents passed through that film leads to varying effective electrical resistance in the film in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the field and the current direction therein are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance in such a magnetoresistive ferromagnetic film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance has a magnitude characteristic that follows the square of the cosine of that angle.
Operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film. Such an axis comes about in the film because of an anisotropy therein typically resulting from depositing the film during fabrication in the presence of an external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of the device having this resulting film, such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film""s easy axis. The state of the magnetization vector in such a film can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as a memory cell.
In contrast to this arrangement, the resistance in the plane of a ferromagnetic thin-film is isotropic for the giant magnetoresistive effect rather than depending on the direction of the sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect involves a change in the electrical resistance of the structure thought to come about from the passage of conduction electrons between the ferromagnetic layers in the xe2x80x9csandwichxe2x80x9d structure, or superlattice structure, through the separating nonmagnetic layers with the resulting scattering occurring at the layer interfaces, and in the ferromagnetic layers, being dependent on the electron spins. The magnetization dependant component of the resistance in connection with this effect varies as the sine of the absolute value of half the angle between the magnetization vectors in the ferromagnetic thin-films provided on either side of an intermediate nonmagnetic layer. The electrical resistance in the giant magnetoresistance effect through the xe2x80x9csandwichxe2x80x9d or superlattice structure is lower if the magnetizations in the separated ferromagnetic thin-films are parallel and oriented in the same direction than it is if these magnetizations are antiparallel, i.e. oriented in opposing or partially opposing directions. Further, the anisotropic magnetoresistive effect in very thin films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas a significant giant magnetoresistive effect is obtained only in very thin films. Nevertheless, the anisotropic magnetoresistive effect remains present in the films used in giant magnetoresistive effect structures.
A memory cell based on the xe2x80x9cgiant magnetoresistive effectxe2x80x9d can be provided by having one of the ferromagnetic layers in the xe2x80x9csandwichxe2x80x9d construction being prevented from switching the magnetization direction therein from pointing along the easy axis therein in one to the opposite direction in the presence of suitable externally applied magnetic fields while permitting the remaining ferromagnetic layer to be free to do so in the same externally applied fields. In one such arrangement, a xe2x80x9cspin-valvexe2x80x9d structure is formed by providing an antiferromagnetic layer on the ferromagnetic layer that is to be prevented from switching in the externally applied fields to xe2x80x9cpinxe2x80x9d its magnetization direction in a selected direction. In an alternative arrangement often termed a xe2x80x9cpseudo-spin valvexe2x80x9d structure, the ferromagnetic layer that is to be prevented from switching in the externally applied fields is made sufficiently thicker than the free ferromagnetic layer so that it does not switch in those external fields provided to switch the free layer.
Thus, a digital data memory cell based on the use of structures exhibiting the giant magnetoresistive effect is attractive as compared to structures based on use of an anisotropic magnetoresistive effect because of the larger signals obtainable in information retrieval operations with respect to such cells. Such larger magnitude signals are easier to detect without error in the presence of noise thereby leading to less critical requirements on the retrieval operation circuitry.
An alternative digital data bit storage and retrieval memory cell suited for fabrication with submicron dimensions can be fabricated that provides rapid retrievals of bit data stored therein and a low power dissipation memory through use of a cell structure that has a spin dependent tunneling junction (SDTJ), or magnetoresistive tunnel junction (MTJ), device therein based on a pair of ferromagnetic thin-film layers having an electrical insulator layer therebetween of sufficient thinness to allow tunneling currents therethrough. This memory cell can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the magnetic memory films present in such a xe2x80x9csandwichxe2x80x9d structure on either side of an intermediate nonmagnetic layer where such ferromagnetic films may be composite films, but this intermediate nonmagnetic layer conducts electrical current therethrough based primarily on the quantum electrodynamic effect xe2x80x9ctunnelingxe2x80x9d current mentioned above.
This xe2x80x9ctunnelingxe2x80x9d current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers, the latter being set by the layer magnetization directions to provide a xe2x80x9cmagnetic valve effectxe2x80x9d. Such an effect results in an effective resistance or conductance characterizing this intermediate layer with respect to the xe2x80x9ctunnelingxe2x80x9d current therethrough. In addition, an antiferromagnetic layer against one of the ferromagnetic layers is used in such a cell to provide different magnetization switching thresholds between that ferromagnetic layer and the other by fixing, or xe2x80x9cpinningxe2x80x9d, the magnetization direction for the adjacent ferromagnetic layer while leaving the other free to respond to externally applied fields. Such devices may be provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such memory cell device and the operating circuitry therefor.
A xe2x80x9csandwichxe2x80x9d structure for such a memory cell, based on having an intermediate thin layer of a nonmagnetic, dielectric separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned, exhibits the xe2x80x9cmagnetic valve effectxe2x80x9d if the materials for the ferromagnetic thin-films and the intermediate layers are properly selected and have sufficiently small thicknesses. The resulting xe2x80x9cmagnetic valve effectxe2x80x9d can yield a response which can be several times in magnitude greater than that due to the xe2x80x9cgiant magnetoresistive effectxe2x80x9d in a similar sized cell structure.
An example of a two state magnetoresistive device structure that is generally common to both of these kinds of memory cells is the xe2x80x9cpinned sandwichxe2x80x9d structure shown in the layer diagram of FIGS. 1A and 1B where the section line of FIG. 1B defines the view shown in FIG. 1A. This layer diagram gives an indication of the structural layers, but is not a true cross section view in that many dimensions there are exaggerated or reduced relative to one another for purposes of clarity.
A substrate, 2, supports an interconnection structure, 3, as the bottom contact electrode to a magnetic material (ferromagnetic material) free layer, 4, (meaning its magnetization is relatively free to be rotated to an alternative orientation) that is separated by a nonmagnetic material spacer layer, 5, from a magnetic material (ferromagnetic material) relatively fixed layer, 6, (meaning its magnetization is much less free to be rotated to an alternative orientation, i.e. xe2x80x9cpinnedxe2x80x9d). This xe2x80x9cpinningxe2x80x9d of layer 6 is provided by a further magnetic material layer, 7, the xe2x80x9cpinningxe2x80x9d layer, that is of an antiferromagnetic material which is magnetically coupled to pinned layer 6 and thereby serves to make this two layer pinned structure relatively resistant to rotation of its initial joint magnetization direction in the presence of moderate external applied magnetic fields. An aluminum cap layer, 8, serves as the device top contact electrode providing a conductive path to a further interconnection, 9.
If spacer layer 5 is an electrical conductor, such as Cu, then the structure will exhibit the giant magnetoresistive (GMR) effect and be termed a xe2x80x9cspin valvexe2x80x9d. If spacer layer 5 is an electrical insulator, such as Al2O3, that is sufficiently thin, then the device will exhibit the spin dependent tunneling effect and be termed a xe2x80x9cmagnetic tunnel junctionxe2x80x9d. In either situation, the electrical resistance of the device is typically higher when the magnetizations of the free and fixed layers on either side of the spacer layer are oriented antiparallel to one another, and is lower when these magnetizations are oriented parallel to one another. The electrical resistance versus external applied magnetic field response characteristic for a spin valve that is measured for sense current being established across the magnetic material layers with the conductive layer therebetween is greater in terms of fractional change than that characteristic measured for the sense current established parallel to these layers because the entire collection of spins in the sense current electrons is forced to interact with both magnetic material layers for the sense current being established across these layers but only a fraction of these electrons interact with both layers for sense currents established parallel thereto.
Plots of the high externally applied magnetic field range and the low externally applied magnetic field range response characteristics of a typical spin valve are shown in the graphs of FIGS. 2A and 2B, respectively. The device resistance versus externally applied magnetic field response characteristics of a magnetic tunnel junction are qualitatively similar. However, the magnitudes of the resistance values and the resistance change values may be quite different. FIG. 2B shows that at moderately high positive externally applied magnetic fields the device resistance is largest, corresponding to the antiparallel alignment of the magnetizations of free and fixed layers 4 and 6; and the device resistance is smallest for moderately high negative externally applied magnetic fields, corresponding to the parallel alignment of the magnetizations of free and fixed layers 4 and 6.
As stated above, operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector with respect to the easy axis in the ferromagnetic films of these various kinds of memory cell devices, particularly the free layers. Such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the layer magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the easy axis of the layer, the state of the cell determining the value of the binary bit being stored therein. One of the difficulties in such memories is the need to provide memory cells therein that have extremely uniform switching thresholds and adequate resistance to unavoidable interjected magnetic field disturbances in the typical memory cell state selection scheme used. This externally applied operating fields scheme is based on selective externally imposed magnetic fields provided by selectively directing electrical currents over or through sequences of such cells thereby giving rise to such magnetic fields so that selection of a cell occurs through coincident presences of such fields at that cell.
In such a coincident current selection arrangement, only that cell in the vicinity of the crossing location, or intersection, of these two paths experience sufficient magnetic field intensities due to the summing of the fields due to these two currents to cause such a magnetic state change therein. Cells in the array that are located far away from both of these two current paths are not significantly affected by the magnetic fields generated by such currents in the paths because such fields diminish in intensity with distance from the source thereof. Cells, however, located in relatively close proximity to one, but not two, of these two paths do experience more significant magnetic fields thereabout, and those immediately in or adjacent to one such path experience sufficient field intensities to be considered as being xe2x80x9chalf-selectedxe2x80x9d by the presence of current in that path intended to participate in fully selecting a different cell along that path at the intersection with the other path on which a selection current is present. Half-selection means that a bit is affected by magnetic fields from the current through one path but not another. Such a coincident interjected magnetic fields memory cell state selection scheme is very desirable in that an individual switch, such as that provided by a transistor, is not needed for every memory cell, but the limitations this selection mode imposes on the uniformity of switching thresholds for each memory cell in a memory make the production of high yields difficult.
As such magnetic thin-film memory cells are made smaller to thereby increase the cell density over the surface of the substrate on which they are disposed, the resulting cells become more subject to magnetic state, or data, upsets due to thermal fluctuations occurring in the materials therein. The depth of the energy well in the magnetic material of such cells can be approximated as Hweff*Ms*Volume, where Hweff is half the effective restoration magnetic field attempting to maintain the current magnetic state following perturbations thereto and so effectively providing the energy well depth, Ms is the saturation magnetization of the magnetic material in the cell, and Volume is the volume of the magnetic material in the cell. In conventional cells, Hweff is provided by shape anisotropy or anisotropy due to the material properties of the cell magnetic material, or both. Typically, the value of Hweff in these cells is less than 100 Oe.
Plotting the magnetostatic energy of a data storage cell magnetic material layer versus the angle between the magnetization and the easy axis of that layer, an energy minimum is seen in the result at the angular value of zero or, with this angle designated as xcex8, at xcex8=0 as shown in the graph of FIG. 3. This minimum, having on either side thereof in this plot an energy maximum, that is energy maxima at xcex8=+90xc2x0 and xcex8=xe2x88x9290xc2x0, is the xe2x80x9cenergy wellxe2x80x9d. The depth of the energy well when no external magnetic fields are applied is simply the difference between the energy minimum and maxima. The value of this energy well can be calculated as follows:
E=xc2xd sin2xcex8|{right arrow over (M)}|HkV
where {right arrow over (M)} is the magnetization, Hk is the anisotropy field, V is the volume, and xcex8 is the angle of {right arrow over (M)} from the easy axis. The magnetization orientation will tend to orient to minimize the magnetostatic energy; i.e,.xcex8 will tend toward zero degrees.
The graph of FIG. 4 shows a Stoner-Wohlfarth switching threshold plot, a portion of an asteroid, and reasonable values of the word and sense fields to provide adequate margins for a memory employing coincident current selection. The solid curve in the figure represents the total field required to cause a bit magnetization to switch from one to the other of two stable states. The total field is the vector sum of the word magnetic field {right arrow over (H)}w due to current provided in an adjacent word line, and the sense magnetic field {right arrow over (H)}s due to current provided through the cell which currents are typically applied along current paths following the two orthogonal axes in the plane of the cell array. The Gaussian curve portion shown in the middle of the plot is representative of the distribution of cell applied magnetic field switching threshold values in an array of real memory cells. The memory array design, then, must account for the varying cell switching thresholds encountered in view of this distribution. As illustrated in the figure, design values for the word and sense fields are about xc2xd the value of Hk. The remaining energy well depth of those cells half-selected is about xc2xc their non-selected depth. This can be shown through calculating the well depth with half selection magnetic fields both present and absent.
The energy expression above, when modified to include the effects of {right arrow over (H)}w and {right arrow over (H)}s, becomes
xe2x80x83E=xc2xd sin2xcex8|{right arrow over (M)}|HkVxe2x88x92|{right arrow over (M)}||{right arrow over (H)}s|V sinxcex8+|{right arrow over (M)}||{right arrow over (H)}w|V cosxcex8
Here we assume that {right arrow over (H)}w is parallel to the effective easy axis while {right arrow over (H)}s is perpendicular to this axis. The easy axis is parallel to Hk.
If a half-select word field is applied (i.e. |{right arrow over (H)}w|=xc2xdHk and |{right arrow over (H)}s|=0), the energy expression becomes:
E=xc2xd sin2xcex8|{right arrow over (M)}|HkV+|{right arrow over (M)}||{right arrow over (H)}w|V cosxcex8,
where the second term is the energy due to the applied word field. If a half-select sense field is applied (i.e. |{right arrow over (H)}s|=xc2xdHk and |{right arrow over (H)}w|=0), the energy expression becomes:
E=xc2xd sin2xcex8|{right arrow over (M)}|HkVxe2x88x92|{right arrow over (M)}||{right arrow over (H)}s|V sinxcex8,
where the second term is the energy due to the applied sense field.
These two equations are plotted in the graphs of FIGS. 5A and 5B. In both cases, the well depth has been reduced by a factor of four, from xc2xd MHkV to xe2x85x9 MHkV . A physical memory may be designed with slightly different parameters. However, the important factor is the smallest energy well depth for a half-selected cell. The design objective is to ensure that the memory cells are magnetically stable during the data storing, or magnetic state switching, procedure that is repeatedly undertaken with respect to other cells. However, the trade-off between thermal stability and magnetic stability is a serious problem when the total magnetic volume of bits is less than about 105 nm3.
A ferromagnetic layer and an antiferromagnetic layer can be deposited in succession so they are in contact with one another with the result that relatively large interatomic forces occur aligning electron spins (parallel for ferromagnetism and antiparallel for antiferromagnetism). These coupling forces at the interface between these layers can be such that the magnetization of the ferromagnetic layer is restored to its initial direction prior to being subjected to external magnetic fields even after very large external magnetic fields are subsequently applied thereto. Such external magnetic fields can be 1000 Oe or more, and the magnetization of the ferromagnetic layer will still be restored to its initial direction. Thus, if such an antiferromagnetic layer is provided in contact with a ferromagnetic layer in a memory cell so that relatively large coupling occurs therebetween, the energy well depth for a small memory cell can be greatly increased. Such an arrangement can increase the potential density of memory cells by more than a factor of 10 through permitting the cell dimensions to go from about 0.2 xcexcm minimum dimensions to approximately 0.05 xcexcm dimensions.
A film structure which exhibits even better resistance to the effects of large externally applied magnetic fields is provided by a compound ferromagnetic thin-film layer with an antiferromagnetic layer. This compound ferromagnetic thin-film layer is provided to have a net layer magnetization that, when fixed in orientation in the finally formed structure, will resist rotation of its magnetization so that the magnetization of this compound ferromagnetic thin-film layer will appear fixed in its orientation in the device, i.e. xe2x80x9cpinnedxe2x80x9d in a direction relative to the finally formed structure.
This compound ferromagnetic thin-film layer is formed by depositing a ferromagnetic layer to perhaps a thickness of 40 xc3x85 which is deposited in the presence of an easy axis direction determination magnetic field, then a nonmagnetic layer of ruthenium (no orienting magnetic field needed in this instance) to provide a Ru antiferromagnetic coupling layer of 9 xc3x85 thickness. Thereafter, another ferromagnetic layer is deposited to a thickness of 40 xc3x85 again in the presence of an easy axis direction determination magnetic field aligned as was the field for the first ferromagnetic layer. The resulting compound ferromagnetic layer has materials with high spin polarization in its outer layers due to the use of high magnetic induction ferromagnetic material therein, but has little net magnetic moment because of the Ru layer provided therebetween which strongly antiferromagnetically couples these outer layers through primarily exchange coupling (some magnetostatic coupling also present)so that the magnetizations of each are pointed in opposite directions. Thus, this layer is relatively insensitive to externally applied fields and contributes little to the spatial fields thereabout. However, the magnetization direction in this composite layer by itself is not very strongly fixed in any direction because of the relatively weak anisotropy exhibited by the ferromagnetic layers.
Thus, a further antiferromagnetic material xe2x80x9cpinningxe2x80x9d layer exhibiting a substantial magnetic anisotropy must be deposited on the last ferromagnetic layer to strongly set the magnetization direction of the compound layer. Such an antiferromagnetic layer has a strongly fixed magnetization direction which, through exchange coupling to the last ferromagnetic layer on which it is deposited, strongly fixes the direction of magnetization of that layer also, and so that of the first ferromagnetic layer through the Ru layer. The result is an antiferromagnetic layer coupled strongly to the compound layer. Hence, an antiferromagnetic pinning layer is deposited on the last ferromagnetic layer to a thickness of 100 xc3x85 or more in the presence of a magnetization axis determination magnetic field aligned with the fields used in forming the two ferromagnetic layers.
If this compound ferromagnetic layer with the antiferromagnetic layer thereon is provided across an electrically conductive layer of perhaps 25 xc3x85 thickness from a further ferromagnetic layer of again 40 xc3x85 thickness, a good xe2x80x9cspin-valvexe2x80x9d magnetoresistive memory cell is formed in which this last ferromagnetic layer is the xe2x80x9cfreexe2x80x9d layer which can have its magnetization changed to be either parallel or antiparallel to the firmly fixed magnetization direction of the nearest ferromagnetic layer in the compound ferromagnetic layer to select one of the possible the cell magnetization states (the different states resulting in different cell electrical resistances). This can be accomplished through providing a sufficiently large storage electrical current which will flow primarily through the relatively thick conductive layer between the compound ferromagnetic layer and the xe2x80x9cfreexe2x80x9d layer (although some of this current will also pass through these latter two layers also even though being substantially shunted around by the conductive layer). An external magnetic field directed along the storage current path can also be provided through an appropriately positioned current strap to xe2x80x9ctipxe2x80x9d the magnetization of the xe2x80x9cfreexe2x80x9d layer to reduce the magnitude needed for the storage current to rotate the xe2x80x9cfreexe2x80x9d layer magnetization. A smaller retrieval electrical current can be directed along the cell current path used for the storage current primarily through the conductive layer between the compound ferromagnetic layer and the xe2x80x9cfreexe2x80x9d layer (though again some of this current will also pass through these latter two layers also despite the substantial conductive layer shunting effect).
This common use of the compound ferromagnetic layer with an antiferromagnetic layer thereon is based on its resistance to alteration of its magnetization direction by externally applied magnetic fields. Omitting the antiferromagnetic layer reduces the ability to set the direction of the magnetization in the compound ferromagnetic layer, but whatever magnetization direction results in the compound ferromagnetic layer in the circumstance of no antiferromagnetic layer being present is still, as indicated above, quite insensitive to externally applied magnetic fields if the two ferromagnetic layers therein are well matched in responding to such external fields. This is true since the effect of an external field on one ferromagnetic layer is directly opposed by the effect on the other because of their magnetizations being held strictly antiparallel to one another by the Ru layer therebetween. Thus, use of a compound ferromagnetic layer without an antiferromagnetic layer thereon would also result in the energy well depth for a small memory cell based on this structure being substantially increased due to the demagnetization fields in each ferromagnetic layer being maintained in directions to approximately cancel one another.
On the other hand, structures based on two ferromagnetic layers with a thin layer of Ru positioned therebetween would seem unsuited to serve as the storage structure for a memory cell because external magnetic fields of magnitudes reasonably generated by currents available in a memory cell array (several tens of Oe) fabricated with monolithic integrated circuit fabrication methods cannot cause such a structure to have the ferromagnetic layers therein switch their magnetizations between alternative magnetization directions (resulting upon removal of those external fields). That is, such fields will not cause switching between alternative magnetic states as ways of representing alternative stored values, typically binary values. However, such a storage structure is desirable if switchable because of the absence of demagnetizing fields therein due to the maintained antiparallel magnetizations in the ferromagnetic layers thereof.
The present invention provides a ferromagnetic thin-film based digital memory having a plurality of bit structures electrically interconnected with information storage and retrieval circuitry. Each of these bit structures formed of a relative orientation maintenance intermediate layer having two major surfaces on opposite sides thereof with a pair of memory films of an anisotropic ferromagnetic material each on a corresponding one of said relative orientation maintenance intermediate layer major surfaces. The relative orientation maintenance intermediate layer is of a material and a selected thickness so as to maintain magnetizations of said memory films oriented in substantially opposite directions. The pair of memory films have magnetic moments sufficiently similar to each other so as to substantially maintain a magnetization state, or magnetization orientation primarily aligned with one easy axis direction in the films, despite magnetic fields occurring thereabout due to changes of magnetization states, or magnetization orientations from being aligned in primarily in one direction along the film easy axis to being aligned primarily in the opposite direction, of those said pairs of memory films in others of the plurality of bit structures when being operated or directed to do so by the information storage and retrieval circuitry. Interconnection electrodes pairs are each in electrically conductive contact with a corresponding one of the bit structures on substantially opposite sides thereof near or at substantially opposite edges of the relative orientation maintenance intermediate layer major surface with at least one member of a said interconnection electrode pair being electrically coupled to said information storage and retrieval circuitry. A magnetization reference direction layer, able to maintain relatively well its magnetization orientation, can be provided across a nonmagnetic layer from one of the pair of memory films with the nonmagnetic layer being either electrically conductive or insulative.
In a further embodiment, there is provided another ferromagnetic thin-film based digital memory having a plurality of bit structures electrically interconnected with information storage and retrieval circuitry. Each of these bit structures formed of a relative orientation maintenance intermediate layer having two major surfaces on opposite sides thereof with a memory film of an anisotropic ferromagnetic material on each of said relative orientation maintenance intermediate layer major surfaces. The relative orientation maintenance intermediate layer is of a material and a selected thickness so as to maintain magnetizations of said memory film, adjacent each of said intermediate layer major surfaces, oriented in substantially opposite directions. A nonmagnetic intermediate layer of an electrically insulative material is provided on the memory film across from one of the relative orientation maintenance intermediate layer major surfaces with the nonmagnetic intermediate layer having a major surface on a side thereof opposite the memory film. A magnetization reference layer is on the major surface of the nonmagnetic intermediate layer having a substantially fixed magnetization direction. Interconnection electrodes pairs are each in electrically conductive contact with a corresponding one of the bit structures on substantially opposite sides thereof near or at substantially opposite edges of the relative orientation maintenance intermediate layer major surface with at least one member of a said interconnection electrode pair being electrically coupled to said information storage and retrieval circuitry.